A geosynchronous satellite (GEO) is a satellite flying in a geosynchronous earth orbit, with an orbital period the same as the Earth's rotation period, whereas a low earth orbit (LEO) satellite flies in an orbit around the Earth at an altitude between 160 kilometers or 99 miles (with an orbital period of about 88 minutes) and 2,000 kilometers or 1,200 miles (with an orbital period of about 127 minutes). Further, in a GEO satellite system, the beam footprint on the Earth is generally fixed based on the satellite antenna configuration. Unlike a GEO satellite, a LEO satellite constellation provides a unique mode of operation and associated challenges, where the footprint of each satellite beam on the Earth is constantly moving. The speed of a LEO satellite as observed from a location on the earth is high, which results in a large satellite motion induced Doppler effect. This large Doppler results in (i) a significant offset in the center frequency of the received signal, (ii) a scaling of the time and the frequency axes that causes the so-called Doppler-induced rubber-band effect (both the symbol or chip durations and the signal bandwidth are stretched or compressed as a result), and (iii) a large rate of temporal variation of the received symbol and frame markers. As a result of this Doppler effect, a LEO satellite system presents unique challenges, for example, in the area of network level synchronization. The LEO system Doppler component, however, exhibits a predictable and deterministic nature, but current LEO systems fail to take advantage of this predictable and deterministic nature to simplify the delay and Doppler domain uncertainty ranges introduced by the LEO system technology, which the physical layer receivers must be designed to resolve.
Accordingly, there is a need for an improved synchronization approach in a LEO satellite system, and a further need for such a synchronization approach that takes advantage of the predictable and deterministic nature of the LEO Doppler component, and thereby simplifies the delay and the Doppler domain uncertainty ranges that the physical layer receivers have to resolve.